I started this blog back in last December, so I didn't bother about the spam problem at the beginning.
But last month I started getting a lot of trackback spam, some days even 30-40 per day: I guess this is the new way of blog spam since comments are being filtered by all the CAPTCHA controls or other dictionary based comment filters.
I decided to give Akismet a try: and in the first week after I enabled Akismet I have no more spam on my blog. All the trackbacks are being checked against the Akismet web service and eventually marked as spam.
Their stats say that 95% of all comments are spam: I didn't experienced that number of spam comments and trackbacks, but probably it depends on the topic and audience of the blog, and also, probably their stats are a bit biased because their service is used by blogger with problems with spam.
If you have Subtext, enabling Akismet is pretty easy: just get a personal key (which involves creating an account on worpress.com) and insert it in the comment configuration page of your blog administration.
If you want to add Akismet support into your application there is an opensource Akismet .NET API hosted on CodePlex.